The present invention in general relates to an electric discharge machining apparatus that generates an electric discharge between a workpiece and an electrode so as to carry out machining with respect to the workpiece. More particularly, this invention relates to an electric discharge machining apparatus which can perform machining at high speed.
As a conventional electric discharge machining apparatus, there have been known a wire electric discharge machine carrying out wire electric discharge machining and a die sinking electric discharge machine carrying out die sinking electric discharge machining. In wire electric discharge machining, a conductive wire is used as an electrode so as to carry out machining; on the other hand, in die sinking electric discharge machining, an electrode having various shapes is used so as to carry out machining. FIG. 23 is a view schematically showing a construction of a conventional wire electric discharge machine.
The conventional wire electric discharge machine includes: a conductive wire 51 used as an electrode; a voltage applying circuit 53 for applying a rectangular voltage pulse between the wire 51 and a workpiece 52; a feeder cable 54a for connecting the workpiece 52 and the voltage applying circuit 53; a feeder cable 54b for connecting the wire 51 and the voltage applying circuit 53, and a feeder terminal 55. Further, the conventional wire electric discharge machine includes: a feed reel 56 for feeding the wire 51 to the workpiece 52 side; a winding reel 57 for winding up the fed wire 51; a brake 58 for stopping feed and winding of the wire 51; a winding roller 59 for feeding the wire 51 to the winding reel 57; a cross table 60 for fixing the workpiece 52, and an X-axis motor 61 for moving the cross table 60 to a predetermined X-axis direction.
Moreover, the conventional wire electric discharge machine includes: a Y-axis motor 62 for moving the cross table 60 to a Y-axis direction perpendicular to the X-axis direction; a servo circuit 64 for driving the X-axis motor 61 and the Y-axis motor 62 via motor control cables 63a and 63b; a control circuit 65, which outputs a control signal to the servo circuit 64, and moves the cross table 60 and the workpiece 52 so as to control a machining position. Further, the conventional wire electric discharge machine includes: a working fluid tank 66, which is filled with a working fluid; a pump 67 for pumping a working fluid out of the working fluid tank 66; a working fluid supply pipe 68a for supplying a working fluid from the working fluid tank 66 to the pump 67; a working fluid supply pipe 68b for supplying a working fluid from the pump 67 to the workpiece 52 side, and a guide 69 for feeding the wire 51 to the workpiece 52 side.
In the conventional wire electric discharge machine, the voltage applying circuit 53 applies a rectangular voltage pulse between the wire 51 and the workpiece 52 via the feeder cables 54a and 54b and the feeder terminal 55. By doing so, an electric discharge occurs between the wire 51 and the workpiece 52, and a part of the workpiece 52 is removed by this electric discharge. Subsequently, the workpiece 52 is moved so as to remove a desired portion of the workpiece 52, and thereby, the workpiece is formed into a desired shape. In this case, by the electric discharge, a part of the workpiece 52 is removed while the surface of the wire 51 is being removed. For this reason, when the same portion of the wire 51 is continuously used, the wire 51 wears out. In order to prevent a breakdown of the wire, in the wire electric discharge machine, a portion of the wire 51, where no electric discharge is applied, is fed to the workpiece 52 in succession, and then, machining is carried out while the portion, where electric discharge has been already applied, is wound up in succession.
The feeding of the wire 51 is carried out by the feed reel 56 via the brake 58 and the guide 69; on the other hand, the winding of the wire 51 is carried out by the winding reel 57 via the winding roller 59. The cross table 60 is used to fix the workpiece 52. The X-axis motor 61 and the Y-axis motor 62 two-dimensionally move the cross table 60. An NC device comprising the control circuit 65 and the servo circuit 64 drives the X-axis motor 61 and the Y-axis motor 62 so that the cross table 60 and the workpiece 52 are moved so that machining position is controlled. The working fluid tank 66 is filled with de-ionized water as a working fluid. The pump 67 pumps up a working fluid of the working fluid tank 66 via the working fluid supply pipe 68a, and then, supplies the working fluid to a discharge field via the working fluid supply pipe 68b. 
FIG. 24 is a view showing a configuration of the voltage applying circuit 53 shown in FIG. 23. The voltage applying circuit 53 includes a resistor 74, a switch SW51, a direct current constant voltage source 75, and a switch SW52. More specifically, the resistor 74 has one end connected to the wire 51 via the voltage applying circuit and an inductance 73 included in a current path, and one end of the switch SW51 is connected to the other end of the resistor 74. The direct current constant voltage source 75 is constructed in a manner of connecting the other end of the switch SW51 to a high voltage side, and connecting the workpiece 52 to a low voltage side. The switch SW52 is interposed between the other end of the resistor 74 and the workpiece 52. The direct current constant voltage source 75 generates a predetermined voltage. The resistor 74 is additionally provided for limiting a discharge current. The switch SW51 is a switch for increasing a voltage between the wire 51 and the workpiece 52 (hereinafter, referred simply to as interelectrode); on the other hand, the switch SW52 is a switch for setting a voltage of the inter-electrode to 0V. For example, a field effect transistor (FET) is used as each of these switches.
FIG. 25 is a view showing an operation of a conventional voltage applying circuit 53. In the operation of the voltage applying circuit 53, first, the circuit operation is changed from a state in which the switch SW51 is turned off and the switch SW52 is turned on to a state in which the switch SW51 is turned on and the switch SW52 is turned off. At this time, a voltage rises between the resistor 74 side P51 of the switch SW52 and the workpiece side P52 of the switch SW52, and thus, an interelectrode voltage rises. An interelectrode static capacitance and a value of the inductance 73 are considerably small as compared with the resistor 74; therefore, when the switch SW51 is turned on, an interelectrode voltage rises at a extremely high speed.
After a discharge time lag td (described later) elapsed, an interelectrode discharge starts in the middle of voltage pulse application, and then, a discharge current flows through the interelectrode, and thereby, an interelectrode voltage decreases. Thereafter, when the switch SW51 is turned off and the switch SW52 is turned on, the voltage between P51 and P52 and the interelectrode voltage become 0V, and then, discharge is stopped; as a result, a discharge current becomes 0 ampere. The voltage applying circuit 53 repeats the above operation at a predetermined period, and thereby, intermittently generates a discharge in the interelectrode.
FIG. 26 is a graph showing a relation between a discharge time lag and a discharge probability in a conventional electric discharge machining apparatus. As shown in FIG. 26, a discharge time lag td until a discharge current starts to flow more than a predetermined value after the interelectrode voltage exceeds 10% of the maximum value, means the following time. More specifically, the discharge time lag td is a time adding the minimum formative time lag tf required for starting a discharge and a probability time lag ts that stochastically varies in its length for each discharge together. Namely, even if an interelectrode distance (hereinafter, referred to as gap interval) and physical conditions such as voltage applied to the interelectrode are the same, the discharge time lag td for each discharge is not kept at a constant value, and varies in a predetermined range.
FIG. 27 is a graph showing a relation between a discharge time lag, a discharge probability and a gap interval in the conventional electric discharge machining apparatus. FIG. 27 shows a discharge time lag td when a voltage applied to the interelectrode is fixed to 80 volts, and a gap interval is set to each of 5 xcexcm, 8 xcexcm and 10 xcexcm. As shown in FIG. 27, when the voltage applied to the interelectrode is kept constant and the gap interval is changed, when the gap interval becomes wider, a probability that the discharge time lag td becomes longer, becomes high, and then, time-out comes; as a result, sometimes no discharge occurs. On the other hand, when the gap interval becomes smaller, a probability that the discharge time lag td becomes shorter becomes high.
The gap interval is not always kept constant, and its value varies after and before average by a vibration of the wire 51 and unevenness of the workpiece 52. Therefore, when the gap interval is made too small, the wire 51 and the workpiece 52 short-circuit by variation of the gap interval; as a result, sometimes no discharge occurs. Moreover, when the wire 51 and the workpiece 52 short-circuit, sometimes the wire 51 wears out. Thus, the gap interval is securely kept to a predetermined value or more so that no problems as described above arises. In addition, when the gap interval is made small so as to make high interelectrode field strength, this increases a probability that a discharge is continuously made at the same portion; a so-called concentrated discharge occurs.
FIG. 28 is a graph showing a relation between a discharge time lag, a discharge probability and an applied voltage in the conventional electric discharge machining apparatus. FIG. 28 shows a discharge time lag td when a gap interval is fixed to 5 xcexcm, and a voltage applied to the interelectrode is set to each of 80V and 100V. A shown in FIG. 28, when the gap interval is kept constant and a voltage applied to the interelectrode is changed, when the voltage applied to the interelectrode becomes lower, a probability that the discharge time lag td becomes longer, becomes high. Then, time-out comes; as a result, sometimes no discharge occurs. On the other hand, when the voltage applied to the interelectrode becomes higher, a probability that the discharge time lag td becomes shorter becomes high. Moreover, when the voltage applied to the interelectrode is set high so as to make strong the electric field strength, a probability that a concentrated discharge occurs becomes high.
This voltage applying circuit applies only positive voltage having the same polarity to the interelectrode. When only voltage having the same polarity is applied to the interelectrode, there is a problem that the workpiece 52 or the like is corroded and deteriorated by an electrolytic effect. For this reason, in place of the voltage applying circuit 53 applying only voltage having the same polarity to the interelectrode, another voltage applying circuit, which generates positive and negative voltage pulse and applies it to the interelectrode, is used, and thereby, it is possible to reduce corrosion and deterioration of the workpiece 52 or the like.
FIG. 29 is a view showing a configuration of a conventional another voltage applying circuit. In the voltage applying circuit, in order to improve a machining speed and a final surface finish accuracy, machining is divided into several steps, and then, first, high speed rough machining is carried out, and thereafter, finishing after two-time machining is carried out. The voltage applying circuit includes a first voltage applying circuit 100 used for roughing and finishing, and a second voltage applying circuit 101 used for roughing. The first voltage applying circuit 100 includes a direct current constant voltage source 83, a capacitor 85, a resistor 97, FET 87, FET 88, FET 89 and FET 90. More specifically, the direct current constant voltage source 83 generates a predetermined voltage, the capacitor 85 has both terminals connected to both terminals of the direct current constant voltage source 83, and the resistor 97 has one end connecting the workpiece 52 via an inductance 98 included in the first voltage applying circuit and a current path. The FET 87 is interposed between a high voltage side of the direct current constant voltage source 83 and the wire 51, and the FET 88 is interposed between a low voltage side of the direct current constant voltage source 83 and the wire 51. The FET 89 is interposed between the high voltage side of the direct current constant voltage source 83 and the other end of the resistor 97, and the FET 90 is interposed between the low voltage side of the direct current constant voltage source 83 and the other end of the resistor 97.
The resistor 97 is additionally provided in order to limit a discharge current. The FET 87 to FET 90 constitute a full bridge circuit, and the FET 87 and the FET 90 are turned on at the same time, and thereby, a positive rectangular voltage pulse is applied to the wire 51 side. Further, the FET 88 and the FET 89 are turned on at the same time, and thereby, a negative rectangular voltage pulse is applied to the wire 51 side. Furthermore, the FET 88 and the FET 90 are turned on at the same time, and thereby, a voltage on the wire 51 side becomes 0V. The first voltage applying circuit 100 repeatedly turns on the FET 87 and the FET 90, the FET 88 and the FET 90, the FET 88 and the FET 89, and the FET 88 and the FET 90, and then, generates a positive and negative voltage pulse as shown in FIG. 30. FIG. 30 shows a voltage between the wire 51 side P61 of the FET 88 and the FET 90 side P62 of the resistor 97. In roughing, machining is carried out using a positive and negative voltage pulse generated by the first voltage applying circuit 100 without using the second voltage applying circuit 101.
On the other hand, the second voltage applying circuit 101 includes a direct current constant voltage source 84, a capacitor 86, a diode 95 and a diode 96, FET 91 and FET 92 and a diode 93 and a diode 94. More specifically, the direct current constant voltage source 84 generates a predetermined voltage, and the capacitor 86 has both terminals connected to both terminals of the direct current constant voltage source 84. The diode 95 has a cathode connected to the workpiece 52 via an inductance 99 included in the second voltage applying circuit and a current path, and the diode 96 has an anode connected to the wire 51. The FET 91 is interposed between a high voltage side of the direct current constant voltage source 84 and the anode of the diode 95, and the FET 92 is interposed between a low voltage side of the direct current constant voltage source 84 and the cathode of the diode 96. The diode 93 has an anode connected to the low voltage side of the direct current constant voltage source 84 and a cathode connected to the anode of the diode 95. The diode 94 has a cathode connected to the high voltage side of the direct current constant voltage source 84 and an anode connected to the cathode of the diode 96.
The FET 91 and the FET 92 are turned on at the same time, and thereby, a negative rectangular voltage pulse is applied to the wire 51 side. The second voltage applying circuit 101 has low impedance and a large capacitor; therefore, a peak discharge current having a high peak value flows. When the FET 91 and the FET 92 are turned off, a feedback current flows via the diodes 93 and 94 by an energy stored in the inductance 94. In roughing, high-speed machining is carried out using a peak discharge current (main discharge) having a high peak value by the second voltage applying circuit 101. However, in this case, the main discharge has a high peak value; for this reason, an abnormal discharge occurs, and sometimes the wire 51 wears out. In order to solve the problem, the first voltage applying circuit 100 helps the main discharge so as to generate a preliminary discharge for normally making a discharge.
FIG. 31 is a view to explain a conventional operation in roughing. In FIG. 31, a voltage between P61 and P62 is shown, and a voltage between a cathode terminal P63 of the diode 96 and an anode terminal P64 of the diode 95 is shown by a slant line. In roughing, as shown in FIG. 31, voltage application by the second voltage applying circuit 101 is carried out just after a positive voltage is applied by the first voltage applying circuit 100. Further, voltage application by the second voltage applying circuit 101 may be carried out just after a negative voltage is applied by the first voltage applying circuit 100. The second voltage applying circuit 101 applies only negative voltage having the same polarity to the interelectrode. However, a voltage applied to the interelectrode is low; therefore, there is almost no influence of corrosion and deterioration.
FIG. 32 is a view showing a relation between an applied voltage and a discharge current in conventional roughing. As shown in FIG. 32, in roughing, a voltage is applied by the first voltage applying circuit 100, and when a weak preliminary discharge occurs in the interelectrode, the voltage application is changed into voltage application by the second voltage applying circuit 101 as fast as possible so as to generate a strong peak main discharge between the interelectrode. A discharge detecting circuit (not shown) detects a preliminary discharge, and then, informs a control circuit (not shown) about the detection result.
When receiving the notification of detection result that preliminary discharged is started, the control circuit (not shown) controls the second voltage applying circuit 101 so that voltage application for main discharge is started, and simultaneously controls the first voltage applying circuit 100 so that the voltage application for preliminary discharge is stopped. It is desirable that a power exchange time tx until the voltage application for main discharge is started from the preliminary discharge occurs is set so as to become short as much as possible. Moreover, by making short a pulse interval until voltage application for starting the next preliminary discharge is carried out from the preliminary discharge occurs; it is possible to improve a machining speed. However, the more the pulse interval is made short, the more a concentrated discharge is easy to occur.
However, according to the aforesaid technique, a rectangular voltage pulse is used to generate a discharge; for this reason, when a voltage value of the rectangular voltage pulse is high, or when a gap interval is made small by dispersion, an interelectrode field strength becomes higher than a predetermined value. As a result, a problem has arisen such that a concentrated discharge occurs and the electrode wears out. In particular, in the case of carrying out a main discharge and a preliminary discharge, a concentrated discharge easily occurs. In this case, the energy of main discharge is great; for this reason, when a concentrated discharge occurs, the electrode wears out completely. Moreover, when a voltage value of the rectangular voltage pulse is low, or when a gap interval is made large by dispersion, the interelectrode field strength becomes lower than a predetermined value. As a result, a discharge time lag becomes long, and a machining speed becomes slow. In addition, a time-out of discharge comes; for this reason, a problem has arisen such that a discharge mistake generating no discharge increases.
It is an object of this invention to provide an electric discharge machining apparatus which can reduce a generation of concentrated discharge so as to prevent a breakdown of electrode, and can carry out high speed machining while reducing a discharge mistake.
The electric discharge machining apparatus according to one aspect of this invention comprises an electrode for generating an electric discharge between a workpiece and thereby machining the workpiece; and a voltage applying unit which applies a voltage pulse between the electrode and the workpiece. This voltage applying unit applies a voltage pulse which has a rise time longer than a discharge formative time lag when a rectangular voltage pulse is applied when a distance between the workpiece and the electrode is an average value in machining and rises up to the same voltage value as the rectangular voltage pulse.
According to the above-mentioned aspect of this invention, the voltage applying unit applies a voltage pulse between a workpiece and an electrode so as to generate an electric discharge between the workpiece and the electrode. The voltage pulse has a rise time longer than a discharge formative time lag when a rectangular voltage pulse is applied when a distance between the workpiece and the electrode is an average value in machining, and rises up to the same voltage value as the rectangular voltage pulse. By doing so, in accordance with a gap interval, an interelectrode voltage is increased to a dischargeable voltage value, while a normal discharge is started before the interelectrode voltage becomes a voltage value generating a concentrated discharge.
The electric discharge machining apparatus according to another aspect of this invention comprises an electrode for generating an electric discharge between a workpiece and thereby machining the workpiece; a second voltage applying unit which applies a second voltage pulse between the electrode and the workpiece so as to generate a second electric discharge with a second electric current; and a first voltage applying unit which applies a first voltage pulse between the electrode and the workpiece so as to generate a first electric discharge with a first electric current which is larger than the second electric current. The first voltage applying unit applies the first voltage pulse which has a rise time longer than a discharge formative time lag when a rectangular voltage pulse is applied when a distance between the workpiece and the electrode is an average value in machining and rises up to the same voltage value as the rectangular voltage pulse when generating the first electric discharge.
According to the above-mentioned aspect of this invention, the first voltage applying unit applies a voltage pulse between a workpiece and an electrode so as to generate a first electric discharge between the workpiece and the electrode. The voltage pulse has a rise time longer than a discharge formative time lag when a rectangular voltage pulse is applied when a distance between the workpiece and the electrode is an average value in machining, and rises up to the same voltage value as the rectangular voltage pulse. By doing so, in accordance with a gap interval, an interelectrode voltage is increased to a dischargeable voltage value, while a normal discharge is started before the interelectrode voltage becomes a voltage value generating a concentrated discharge.
Further, in the electric discharge machining apparatus, it is preferable that the rise time of the voltage pulse is set to 0.1 micro seconds or more and 100 micro seconds or less.
Thus, the rise time of the voltage pulse is set to 0.1 xcexcs or more and 100 xcexcs or less. By doing so, in accordance with a gap interval, an interelectrode voltage is increased to a dischargeable voltage value, while a normal discharge is started before the interelectrode voltage becomes a voltage value generating a concentrated discharge.
Further, in the electric discharge machining apparatus, it is preferable that the voltage applying unit includes a direct current constant voltage source generating a predetermined voltage; and a capacitor-resistor circuit for dulling a rise of voltage generated by the direct current constant voltage source so as to generate the voltage pulse.
The direct current constant voltage source generates a predetermined voltage, and the capacitor-resistor circuit gets dull a rise of voltage generated by the direct current constant voltage source so as to generate the voltage pulse. Therefore, it is possible to generate a voltage pulse by a simple circuit.
Further, in the electric discharge machining apparatus, it is preferable that the voltage applying unit includes a capacitor which generates the voltage pulse by a voltage between both terminals of the capacitor; and a direct current constant current source for supplying a current to the capacitor until the voltage between both terminals of the capacitor becomes a predetermined value.
The capacitor generates the voltage pulse by a voltage between both terminals of the capacitor, and the direct current constant current source supplies a current to the capacitor until the voltage between both terminals of the capacitor becomes a predetermined value. By doing so, it is possible to generate a voltage pulse having a voltage value rising up in proportional to an applied time.
Further, in the electric discharge machining apparatus, it is preferable that the voltage applying unit includes a first direct current constant voltage source for generating a first predetermined voltage, and raises the voltage pulse to the first voltage; and a second direct current voltage source for generating a second voltage higher than the first voltage, and raises the voltage pulse from the first voltage to the second voltage.
The first direct current constant voltage source generates a first predetermined voltage and raises the voltage pulse to the first voltage, and the second direct current voltage source generates a second voltage higher than the first voltage and raises the voltage pulse from the first voltage to the second voltage. By doing so, the voltage pulse rapidly rises up to the first voltage, and thereafter, the voltage pulse rises up to the second voltage.
Further, in the electric discharge machining apparatus, it is preferable that the voltage applying unit further includes a capacitor-resistor circuit for getting dull the voltage pulse rising from the first voltage to the second voltage.
The capacitor-resistor circuit gets dull the voltage pulse rising from the first voltage to the second voltage. Therefore, it is possible to generate a voltage pulse by a simple circuit.
Further, in the electric discharge machining apparatus, it is preferable that the voltage applying unit includes a capacitor which generates the voltage pulse by a voltage between both terminals of the capacitor; a first direct current constant voltage source for setting the voltage between both terminals of the capacitor to a predetermined first voltage before the voltage pulse application is started; and a direct current constant current source for supplying a current to the capacitor until the voltage between both terminals of the capacitor becomes a second voltage higher than the first voltage after the voltage pulse application is started.
The capacitor generates the voltage pulse by a voltage between both terminals of the capacitor, and the first direct current constant voltage source sets the voltage between both terminals of the capacitor to a predetermined first voltage before the voltage pulse application is started. Further, the direct current constant current source supplies a current to the capacitor until the voltage between both terminals of the capacitor becomes a second voltage higher than the first voltage after the voltage pulse application is started. By doing so, first, the voltage pulse rapidly rises up to the first voltage, and thereafter, the voltage pulse rises up to the second voltage in proportional to an applied time.
Further, in the electric discharge machining apparatus, it is preferable that the first voltage is set to 0V or more and 100V or less, and the second voltage is set to 60V or more and 300V or less.
The first voltage is set to 0V or more and 100V or less, and the second voltage is set to 60V or more and 300V or less. By doing so, in accordance with a gap interval, an interelectrode voltage is increased to a dischargeable voltage value, while a normal discharge is started before the interelectrode voltage becomes a voltage value generating a concentrated discharge.
Other objects and features of this invention will become apparent from the following description with reference to the accompanying drawings.